Set your language code in _config.yml:
lang: en
Theme strings can be translated in _data/translation.yml, copy the current English translation and paste it bellow the Eglish translation, then replace en with you language code that you set in _config.yml and translate the strings.
en:
previous: "Previous"
next: "Next"
related_docs: "Related Docs"
related_posts: "Related Posts"
read_more: "Read more"
written_by: "Written by"
share_twitter: "Share on Twitter"
share_facebook: "Share on Facebook"
search_placeholder: "Search for answers"
search_no_results: "No results found"
mobile_nav_docs: "Docs"
mobile_nav_site: "Menu"
contact_name: "Name"
contact_email: "Email"
contact_subject: "Subject"
contact_message: "Message"
contact_send: "Send"